# Homework Help: Calculating Torque and Tangential force from X,Y Vectors

1. Jan 16, 2005

### Jdo300

Hello all,

I'm have a software program I using which gives me an output the X and Y vector forces at Point P on a disk, and I was wondering what calculations I need to use to convert the forces on Point P to the angular (Tangential?) force on the edge of the disk; I know the position of point P, as well as the size of the disk. I was also wondering how to calculate the amount of torque on the disk from the Vector forces of point P. I drew up a small diagram to help illustrate what I'm trying to do. Any help will be greatly appreciated.

Thanks,
Jason O

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2. Nov 25, 2006

### najia

sorry dear no idea for this because i am very bad at maths.
but still i will try to get the answer for this bye.

3. Nov 26, 2006

### OlderDan

I am assuming P is an arbitrary point on the disk. There will be a vector r from the center of the disk to point P. That vector will have components x and y so in terms of the usual horizontal and vertical axes you have

r = x i + y j
F = F_x i + F_y j

The vector r forms an angle θ with the positive x-axis. In terms of this angle x = r*cosθ and y = r*sinθ. At P the vector F forms an angle φ with the positve x axis. In terms of this angle F_x = F*cosφ and F_y = Fsinφ. The equations for r and F become

r = r*cosθ[ B]i[/B] + r*sinθ j
F = F*cosφ i + F*sinφ j

What you are looking for is a way to express F in terms of components in the direction of r and in the perpendicular direction. If you move P to some other location in your diagram and draw the vectors r and F you should see that there is an angle between r and F that can be expressed in terms of θ and φ. But the force stays at point P. It is not applied at the edge of the wheel. Even if you leave the vectors in terms of x and y components, you can compute the torque from the defining equation

τ = r x F